1. Field of the Invention
The present invention relates to a system for creating a seismic-resistant and fire-resistant head-of-wall structure for a nonload-bearing wall within a building.
2. Description of the Prior Art
Seismic and fire resistance has become of increasing concern in building construction. In the construction of buildings the framework for the walls of a building is formed of horizontal still members at the floor, at the ends of which vertical corner posts support horizontal beams at the ceiling level. Between the corner posts there are upright supports, called studs, laterally spaced, usually at uniform intervals, to provide the necessary interior structural support for the wall.
Historically, the framework of a building wall was formed entirely of wooden members, including wooden studs. In recent years, however, the use of metal studs has gained increased acceptance, especially in the construction of commercial buildings, such as office buildings, schools, and hospitals. Metal building studs are typically formed of ten to twenty gauge galvanized steel. For ease of fabrication the metal studs are formed of sheet metal bent into a generally "C-shaped" cross section. A relatively broad central web is flanked by a pair of narrower side walls that are bent at right angles to the web or base. The edges of the side walls of the metal stud are normally bent over into a plane parallel to and spaced from the plane of the web.
In the conventional construction of an interior building wall an overhead beam having a U-shaped configuration extends along the tops of the studs. The overhead beam is formed with a horizontally disposed web from which a pair of side walls depend vertically on opposite sides of the web. The side walls embrace the sides of the vertical studs so that the upper extremities of the studs extend in a perpendicular manner into the concave, downwardly facing channel formed by the overhead beam. The spacing of the studs along the length of the beam is typically either sixteen or twenty-four inches.
One problem which occurs in any building during an earthquake is that the seismic ground motion from an earthquake introduces both horizontal and vertical undulations in the building. Because of their elongated, vertical lengths, the metal studs in building wall construction are limber enough to flex sufficiently in a lateral direction and thereby resist inelastic deformation during an earthquake. However, vertical undulations that vary the distance between the floor and ceiling in a room during an earthquake are likely to destroy, or at least damage the integrity, of rigid structural joints between vertical metal studs and horizontal sill and overhead beam members between which the studs extend in a building.
Another problem which may occur is the spread of fire from room to room within a building. While the structure of interior building walls is largely formed of fire-resistant gypsum board sheet, fire can pass between the upper edges of the gypsum board and the ceiling. Fire paths are particularly likely to develop if the joints between the metal studs in the walls and the ceiling above have been damaged by prior seismic activity.
To alleviate these problems a seismic and fire resistant wall structure and method was devised. This system is described in U.S. Pat. No. 5,127,203. According to this system the overhead, downwardly facing, U-shaped beam that extends across the top of the upright studs is provided with fire-retardant material on its underside and with vertical sides that have vertically elongated slots defined thereon. These vertically elongated slots are longitudinally spaced at intervals to accommodate the positions of studs within a vertical, nonload-bearing wall. Fasteners extend through the vertically elongated slots in the overhead beams and into the sides of the metal studs. The fasteners, typically sheet metal screws, are tight enough to provide lateral stability at the joints between the studs and the overhead beam, but are not so tight as to prevent relative vertical motion therebetween.
Preferably, standoff washers are provided at the elongated slots. The standoff washers have faces against which the heads of the screws bear and short legs or flanges which project through the vertically elongated slots in the side walls of the overhead beams. These flanges bear against the sides of the studs. The standoff washers function in the manner depicted and described in U.S. Pat. No. 5,467,566, with specific reference to FIG. 8 of that patent.
As vertical undulations from an earthquake are transmitted through the structural components of a nonload-bearing wall as described in U.S. Pat. No. 5,127,203, the elongated, vertical slots through which the stud fasteners extend permit vertical, oscillatory motion to occur between the upper extremities of the studs and the overhead beams of the nonload-bearing walls. As a result, the stud fasteners maintain structural integrity so that the wall remains undamaged and does not require repair following an earthquake.
In a nonload-bearing wall the web of the beam is preferably secured to the ceiling above by screws that extend vertically upwardly through longitudinally elongated slots in the web of the beam and into the structure of the ceiling above.
These screws also preferably employ standoff washers of the type described in U.S. Pat. No. 5,467,566 so that the head-of-wall structure can accommodate limited interstory drift during an earthquake.
A problem which continues to exist in building construction is the difficulty in making a nonload-bearing wall adequately fire resistant. In a typical building construction a ceiling is formed by galvanized steel, fluted decking atop which a layer of concrete is poured to form the floor above. The fluted steel decking may, for example, be fabricated of eighteen gauge galvanized steel. The flutes, or concave, downwardly facing channels defined in the underside of the decking, are typically about three inches deep and about six inches wide.
Interior, nonload-bearing walls often pass transversely across the flutes. The beams at the tops of such walls are attached to the underside of the decking where the decking projects downwardly between the hollow flutes. Openings having cross-sectional areas equal to the areas of the flutes are thereby formed above the beams that are located at the top of nonload-bearing, interior walls. These openings form transverse passageways across the tops of the walls through which fire can travel.
To prevent the spread of fire through the flutes formed by the decking above nonload-bearing, interior walls, fire-resistant insulation is packed in the flute openings created at the tops of the walls by the flutes. This fire-resistant insulation may be applied by spraying it into the flute openings from each side of the wall. When the insulation dries and congeals it clogs the flute openings at the top of the wall.
As long as the insulation remains in the flute openings, they remain blocked and the insulation prevents the spread of fire across the top of the wall. However, when a fire is burning within a building, it generates a considerable amount of smoke which is heated and expands. The smoke causes a great pressure within a room where a fire is burning. It is known that the pressure of smoke from a fire burning within a room literally blasts the fire insulation out of the flute openings atop the wall. When this occurs the fire can thereupon spread to an adjacent room over the top of the wall through the flute openings.
According to present building construction practice fire insulation is held within the tunnel cavities defined by the flutes of the decking by hand cutting the upper edges of the gypsum board wall panels to follow the corrugations of the decking. The wallboard panels forming the sides of the nonload-bearing walls provide a series of projections that block the flute tunnels from the opposite sides of the wall and thereby hold the insulation in place. However, this system for holding the insulation in position is extremely time consuming, laborious, and expensive.
Hand cutting of the upper region of the wall to follow the convolutions of the corrugated, fluted decking is extremely labor intensive. The labor cost in creating a scalloped upper edge at the top of the wallboard adds significantly to the cost of construction of the wall. Moreover, even if a template is used the hand cuts result in significant gaps remaining which must then be caulked. The process of caulking is also an extremely laborious, labor intensive process, particularly when it is necessary to follow the convolutions of the underside of the fluted decking.
Moreover, conventional caulking is not seismic resistant. That is, even if the caulking originally provides an effective barrier to air currents, if the building structure subsequently is subjected to seismic activity, the caulking crumbles and gaps that allow the passage of air currents are opened. When this occurs the wall no longer offers its original resistance to the spread of fire. As a result, it has not heretofore been possible to provide both seismic resistance and fire resistance in interior building walls that will meet the stringent building codes applicable to structures such as schools and hospitals.
While the system of U.S. Pat. No. 5,127,203 does allow limited vertical cycling at the head-of-wall structure, it does not provide any means for retaining the insulation within the flutes of decking above the downwardly facing overhead channel-shaped beams. Also, slotted beams must be stocked having webs of the various different sizes that are used in different building head-of-wall structures. That is, the webs are typically provided in about six different widths varying between two and a half and eight inches.